Cranial Nerves III, IV & VI and Extra-Ocular Muscle Function

A comprehensive set of flashcards covering anatomy, function, clinical testing and common palsies of cranial nerves III, IV and VI, plus key principles of extra-ocular muscle actions and strabismus assessment.

Which three cranial nerves provide motor supply to the extra-ocular muscles and eyelids?

Cranial nerves III (oculomotor), IV (trochlear) and VI (abducens).

Besides motor fibres, what additional autonomic fibres travel with the oculomotor nerve (CN III)?

Parasympathetic fibres responsible for pupillary constriction.

Where is the main oculomotor nucleus located within the midbrain?

Anterior to the periaqueductal grey matter at the level of the superior colliculus.

What is the function of the Edinger-Westphal nucleus?

It is the origin of the parasympathetic fibres that run with CN III to control the sphincter pupillae and ciliary muscles.

Into which two branches does CN III divide after entering the orbit?

A smaller superior branch and a larger inferior branch.

Which structures are innervated by the superior branch of CN III?

Superior rectus, levator palpebrae superioris, and sympathetic fibres to superior tarsal and dilator pupillae muscles.

Which structures are innervated by the inferior branch of CN III?

Medial rectus, inferior rectus, inferior oblique, and pre-ganglionic fibres to the ciliary ganglion for ciliary and sphincter pupillae muscles.

What is unique about the trochlear nerve’s exit from the brainstem?

CN IV is the only cranial nerve to emerge from the posterior (dorsal) surface of the brainstem.

Which single muscle is supplied by the trochlear nerve (CN IV)?

The superior oblique muscle.

List the primary actions of the superior oblique muscle.

Depresses, abducts (laterally deviates) and intorts the eyeball.

Where are the trochlear nuclei situated in relation to the midbrain?

Immediately anterior to the cerebral aqueduct at the level of the inferior colliculus.

From which part of the brainstem does the abducens nerve (CN VI) emerge?

The dorsal pontomedullary junction.

Which muscle does the abducens nerve innervate and what is its action?

The lateral rectus muscle, causing abduction of the eye.

Give three common pathological causes of ptosis.

Oculomotor nerve palsy, Horner’s syndrome, and neuromuscular disorders such as myasthenia gravis.

What pattern is traced when clinically testing eye movements?

An ‘H’ pattern through all axes of gaze.

What is the primary action of the superior rectus muscle?

Elevation of the eyeball (with secondary adduction and medial rotation).

What is the primary action of the inferior rectus muscle?

Depression of the eyeball (with secondary adduction and lateral rotation).

What movement is produced by the medial rectus muscle?

Adduction of the eyeball.

What movement is produced by the lateral rectus muscle?

Abduction of the eyeball.

List the actions of the inferior oblique muscle.

Elevates, abducts and laterally (extorts) rotates the eyeball.

How does a complete oculomotor nerve palsy typically present?

‘Down and out’ eye position, ptosis due to levator palpebrae weakness, and mydriasis from loss of parasympathetic input.

What visual complaint and compensatory head posture are characteristic of trochlear nerve palsy?

Vertical (and torsional) diplopia when looking down; patients often tuck the chin and tilt the head to the opposite side.

How does an abducens nerve palsy manifest clinically?

Convergent squint with horizontal diplopia, worsened when the patient attempts to gaze toward the affected side.

Define strabismus.

A condition in which the two eyes fail to align properly when focusing on an object.

What does a central, symmetrical corneal light reflex indicate during the light reflex test?

Normal ocular alignment (orthotropia).

What is the purpose of the cover test in ocular examination?

To detect a manifest strabismus (heterotropia) by observing fixation shifts when one eye is occluded.

During a cover test, a temporal (outward) shift of the uncovered eye indicates what type of tropia?

Exotropia.

During a cover test, if the uncovered eye moves inferiorly when the other eye is covered, what type of deviation is present?

Hypertropia of the covered eye, requiring a downward shift in the uncovered eye.